I tore down it into pieces while searching for faults. Found broken touch screen flat flex cable.
Also I found flat flex (ribbon) cable with some traces been broken in the top half of the 3DS and that was the culprit of popping sound and shut off.
This flat flex (ribbon) is connecting top screen backlight, 3D control ? and speakers. Any fail in the backlight circuits of top or bottom screen and 3DS will shut off with popping sound.
So it's easy to fix, just order a replacement cable. No way :)
Checked with multimeter and found there are only 2 traces broken.
I took some hair thin enameled wires. I then scraped off flat flex (ribbon) traces to copper at the connector side and soldered wires there.
Also soldered these wires to connector on lcd side.
And it works :)
I then glued wires to the flat flex cable with tape.
Putting all that flat flex (ribbon) cables back through the hinge was huge pain though.